It has been reported that the API is failing when the URL is invalid. When you use the URL of a not real image the API try to do the local upload and shouts the base64 error.
The hotfix for this bug can be found here: http://chevereto.com/community/threads/2-2-2-fix-for-api-issue-with-fake-url.2405/
The hotfix for this bug can be found here: http://chevereto.com/community/threads/2-2-2-fix-for-api-issue-with-fake-url.2405/